The feasibility of preparing activated carbon from Eichornia crassipes by chemical activation was investigated. Batch experiments were carried out for the sorption of Methylene Blue (MB) and Rhodamine B (RB) onto the prepared activated carbon. The variables studied were initial dye concentration, pH, adsorbent dose, and contact time. Equilibrium data for the adsorption of the dyes onto activated carbon were obtained from batch adsorption experiments. Two-parameter isotherm models including Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich were employed for fitting equilibrium data. Three-parameter isotherm models including Redlich–Peterson, Toth, and Koble–Corrigan models were also employed for fitting the equilibrium data. Linear and non-linear regression methods were used to determine the best fit model to the equilibrium data. It was found that non-linear regression is a better method for determining isotherm parameters. The data were fitted to pseudo-first-order, pseudo-second-order, intraparticle diffusion model, and Elovich equation. The pseudo-second-order model gave the best fit to the equilibrium data as seen from correlation coefficient values. Fourier transform infrared spectroscopy and scanning electron microscopic investigations were carried out to confirm the morphological characteristics of the adsorbent. The prepared activated carbon had greater affinity for adsorbing MB when compared to RB.